A train is just starting to pull out from a station. Suddenly, a man comes running onto the platform, arms flailing, expression desperate as he tries in vain to catch up with the train. What is the cause of the man's behaviour?

I assume this is a real-life situation, where the man is a human adult male, and the train is a real one? (ie not a toy or model) The platform that the man is on is at the station that the train is pulling out of? Is he supposed to be on this train? Does he want to get on the train? Is there someone in the train whose attention he is trying to catch? Does he think someone is in the train? Did he leave something in the train? Anything caught in the train doors? Anyone?